In the oils and gas industry there are producing wells which range from several hundred feet to over 20,000 feet in depth. These wells are narrow, often less than six inches in diameter. In addition, these wells typically have smaller diameter tubes suspended therein. It is often necessary to place chemicals or other apparatus at or near the bottom of the wellbore.
In the case of fluids, the current methods of placing chemicals near or at the bottom of a wellbore are to either push a plug of the fluid down the well, displacing fluid in the wellbore, or to suspend a small diameter tube, often called a coiled tube, down the well, and push the fluid down this coiled tube. Both methods have substantial drawbacks. The chemical may absorb or react with components in the wellbore during transit. Additionally, a substantial quantity of fluid and material suspended therein that was in the wellbore is displaced into the producing formation. In many cases this is not desirable. The use of coiled tubing solves many of these problems, but the cost and risk of hanging a small diameter tube into a wellbore is substantial. Also, corrosive attack of this coiled tubing from injected chemicals is often particularly severe, as the pipe walls tend to be thin compared to the well tubulars.
In the case of solids, solids are often displaced as a slurry into the bottom of the well. Accordingly, many of the problems associated with injecting fluids are present.
There is one method where a particular chemical is coated and then is either placed, allowed to fall, or displaced to a predetermined location in the well. This is soap, used to help foam a well and thereby increase the effect of gas lift. The prior art method of insulating the soap solids during transit down a well is to encase the soap in wax. The encased soap is then put downhole, where wax dissolves as the temperature increases.
In certain wells, particularly deep and hot wells, wax encasement is not particularly useful. For example, it is often desirable to place a foaming soap at the bottom of a deep well. The chemicals take longer to place than for a shallower well. The prior art soap-sticks encased in wax had the obvious shortcoming that waxes often melted prematurely, especially in deep wells where prior production had heated the wellbore. Wells generally follow the geothermal gradient, with hot temperatures downhole and cooler near-ambient temperatures near the ground surface. As a rule, the deeper the well, the hotter the bottomhole temperature.
The presence of hot oil and other chemicals, including encapsulated chemicals, may accelerate the dissolution. Furthermore, as the temperature increases, some temperature-activated or temperature-sensitive chemicals exposed by the degradation of the wax to fluids in the wellbore may react prematurely.
Occasionally, especially when fluids had been circulated to the bottom of the wellbore or displaced into the producing formation, thereby cooling the wellbore, waxes do not melt sufficiently fast to provide chemical needed for a start-up. Wax has the additional problem of lacking mechanical strength, especially as the temperature warmed, and wax might easily be abraded off an encased apparatus.
Larger apparatuses, such as resin coated screen and the like, are often difficult to place because the apparatus is easily damaged during transit down a wellbore. Protective material is not often utilized, because such material eventually becomes problematic trash in the wellbore. Wax provides inadequate protection for such apparatuses.
The last few years have witnessed a drastic increase in research on encapsulated products and methods to produce such products. This is particularly so in the pharmaceutical field. And it is now becoming recognized that encapsulation technology may be useful in many other fields.
U.S. Pat. No. 3,971,852 describes a process for encapsulating various fragrance oils such as oils with citrus and spice odors. The oils are encapsulated in a matrix comprised of polysaccharide and polyhydroxy compounds by converting an emulsion of the fragrance oil droplets in a solution of the matrix ingredients to an encapsulated solid state during a spray drying process. The patent also mentions that miscellaneous chemicals can be encapsulated by the invention method such as drilling fluids and waxes. U.S. Pat. No. 4,269,279 discloses the use of plastic coated magnetic particles in a bead form to increase lubrication for drilling fluids. The encapsulated ferromagnetic particles can be recovered for reuse with a magnetic separator.
An encapsulated invention which has been disclosed for use in boreholes is described in U.S. Pat. No. 4,078,612. The patent describes an explodable material encapsulated in natural gums slurried in a liquid vehicle. The material is pumped into the formation around the wellbore and exploded to increase permeability.
Another U.S. Pat. No. 4,036,301 describes an encapsulated material useful in cementing a well, wherein a cement accelerator is encapsulated in a waxy material and placed within a highly retarded cement slurry. The cement slurry is pumped into the well with the encapsulated accelerator. After proper placement of the cement, circulation is decreased so that the temperature of the cement fluid approaches the bottom hole temperature of the well and melts the encapsulated material, freeing the accelerator which sets the cement.
U.S. Pat. No. 4,362,566 discloses an additional use of encapsulated materials. The patent suggests encapsulating one component of a two or more component adhesive or cement mixture so that the adhesive or cement will not set until the encapsulated component is freed from its reaction-preventive casing.
What is needed is a coating that provided mechanical strength and that can insulate solids, liquids, and even gases during transit down a wellbore, but does not decompose prematurely, and does not leave residue.